Monday, November 4, 2013

Blasting and Earthquakes

Ok, the monstrous blast in the middle of houses in Chicago has got the old memory wheels turning.

How did I get to the conclusion that PGV (peak ground velocity) is much better to use than PGA, which is so enamoured of the thickies around here? It's from blasting! The damage from the big quarry blasts correlates precisely with PGV, since that parameter is directly linked to the maximum stress induced in the rock, and the induced shear strain of the house. Quarry blasts can be huge, with multiple delays, and totally resemble earthquakes, despite what the pooh-poohers may say.

On the other hand, PGA has no physical manifestation whatsoever. It can become infinite as the frequency gets higher, which is the general situation in the East. Those mellow Californians can get away with it because they live on huge soil basins, that always slosh around at 1 Hz or below.

So, great big books have been written about PGV and blasting. In fact, all the lawsuits coming out of this blast will revolve around those books.

So why such a stupid, suicidal blast? Mistakes happen. A new guy on the job, whatever. I hope there was good instrumentation, since this could go in the record books. :)

Update: Things knocked off shelves. That puts it at about 1 cm/s or maybe more. With the old company, I was concerned that Darlington was right beside a huge quarry, and they wanted to put a new nuclear station even closer. I put up instruments, and legally restricted the PGV to 1 mm/s, anything greater would trip the turbines.